A man charged with murdering four women whose first and last names began with the same letter has told a jury that he is not a "monster".

Joseph Naso, 79, nicknamed the "double initial" killer, is serving as his own attorney at his trial.

"I've been waiting two years and two months for this day, to tell my side of the story," Naso, dressed in a dark suit, told the California court in his opening statement.

"I'm not the monster they say killed these women.

"I don't kill people and there's no evidence of that in my writings and photography."

Naso has pleaded not guilty to four counts of first-degree murder with special circumstances.

The victims were all prostitutes with the same first and last initials: 18-year-old Roxene Roggasch in 1977; 22-year-old Carmen Colon in 1978; 38-year-old Pamela Parsons in 1993; and 31-year-old Tracy Tafoya in 1994.

Naso's statement came after prosecutors showed graphic images of the four women's bodies discovered in Northern California, leading some jurors to wipe away tears.

Marin County prosecutor Rosemary Slote called him a "serial rapist and murderer".

Whether the double initials in each victim's name was a coincidence or a plan, investigators have not said.

Prosecutors are seeking the death penalty and were expected to start calling witnesses later today.

Prosecutors say Naso drugged and photographed his unconscious victims, then strangled them and dumped their naked bodies in rural areas.

Ms Slote read from sections of a diary found at Naso's home that appeared to detail rapes of women, while investigators say they found DNA traces linking the defendant to at least two of the victims.